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(1) Originally, any microbial product which, in low concentrations (of the order of µg/ml), can inhibit or kill (susceptible) microorganisms. (2) Currently, 'antibiotic' is generally used to refer to natural semi-synthetic and wholly synthetic antimicrobial compounds which are effective at low concentrations. In aquaculture antibiotics are usually administered as chemotherapeutic agents; in nature many or all antibiotics may have important ecological roles. Within the microorganism a given antibiotic characteristically acts at specific target site(s).